Marin county is considered in the California Floristic Province, a zone of extremely high biodiversity and endemicism. There are numerous ecosystems present, including coastal strand, oak woodland, chaparral and riparian zones.

There are also a considerable number of protected plant and animal species present: fauna include the California Red-legged Frog "Rana aurora draytonii" and California freshwater shrimp, while flora include Marin Dwarf Flax, Hesperolinon congestum; Tiburon Jewelflower, Streptanthus niger; and Tiburon Indian paintbrush, Castilleja neglecta.

A number of watersheds exist in Marin County including Walker Creek, Lagunitas Creek, Miller Creek, and Novato Creek.The Lagunitas Creek Watershed is home to the largest-remaining wild run of coho salmon (Oncorhynchus kisutch) in Central California. These coho are part of the "Central California Coast Evolutionarily Significant Unit," or CCC ESU, and are listed as "endangered" at both the state and federal level.Significant efforts to protect and restore these fish have been underway in the Watershed since the 1980s. Fifty-percent of historical salmon habitat is now behind dams.

Strong efforts are also being made to protect and restore undammed, headwater reaches of this Watershed in the San Geronimo Valley, where upwards of 40% of the Lagunitas salmon spawn each year and where as much as 1/3 of the juvenile salmon (or fry) spend their entire freshwater lives.

The Salmon Protection and Watershed Network ([1]) leads winter tours for the public to learn about and view these spawning salmon, and also leads year-round opportunities for the public to get involved in stream restoration, monitoring spawning and smolt outmigration, juvenile fish rescue and relocation in the summer, and advocacy and policy development.

Demographics

As of the census of 2000, there were 247,289 people, 100,650 households, and 60,691 families residing in the county. The population density was 476 people per square mile (184/km²).

There were 104,990 housing units at an average density of 202 per square mile (78/km²). The racial makeup of the county was 84.03% White, 2.89% Black or African American, 0.43% Native American, 4.53% Asian, 0.16% Pacific Islander, 4.50% from other races, and 3.47% from two or more races. 11.06% of the population were Hispanic or Latino of any race. 11.5% were of Irish, 11.0% English, 10.2% German and 8.4% Italian ancestry according to Census 2000.

80.8% spoke English, 9.6% Spanish, 1.4% French and 1.1% German as their first language.
In 2005 76.9% of Marin County's population was non-Hispanic whites. 12.6% of the population was Latino. 5.3% of the population was Asian and 3.1% was African-American.[citation needed]

In 2000 there were 100,650 households out of which 27.5% had children under the age of 18 living with them, 48.4% were married couples living together, 8.5% had a female householder with no husband present, and 39.7% were non-families. 29.8% of all households were made up of individuals and 9.6% had someone living alone who was 65 years of age or older. The average household size was 2.34 and the average family size was 2.90.

In the county the population was spread out with 20.3% under the age of 18, 5.5% from 18 to 24, 31.0% from 25 to 44, 29.7% from 45 to 64, and 13.5% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 98.2 males. For every 100 females age 18 and over, there were 96.4 males.

The median income for a household in the county was $71,306, and the median income for a family was $88,934. These figures had risen to $83,732 and $104,750 respectively as of 2007. Males had a median income of $61,282 versus $45,448 for females. The per capita income for the county was $44,962. About 4.7% of families and 9.6% of the population were below the poverty line, including 6.9% of those under age 18 and 2.5% of those age 65 or over. Marin County has the second highest median household income in California behind Santa Clara County.

Marin County has the highest per capita income of any county in the United States. This is driven in particular by expensive enclaves in Sausalito, Belvedere, Tiburon, Mill Valley, Larkspur, Corte Madera, Kentfield, Ross, San Anselmo, and portions of San Rafael and Novato where displays of conspicuous consumption, especially luxury cars, are common. The county has the highest density of BMW cars (locally known as 'Basic Marin Wheels') in the United States, according to the local BMW dealership.

The traditionally middle class towns of Fairfax, Novato and San Rafael (where per capita incomes typically paralleled the California state average as late as 1985) also have experienced especially sharp rises in real estate values, due in part to their proximity to the "prestige" address areas. The county's resistance to urban sprawl and its preservation of open space have also had an upward impact on housing prices by reducing the number of new subdivisions built in the area since 1970. The precedent for this was set after a huge development project that would have put a suburb atop the Marin Headlands called Marincello was defeated in court.

The trend of increased affluence has not held true for two neighborhoods in particular, populated almost exclusively by low-income disadvantaged groups: Marin City and the Canal area in San Rafael. Government policies have both forbidden property owners from raising rents and have also subsidized housing prices in these neighborhoods for tenants who do not report incomes higher than 200% of the poverty level on their IRS tax return. Marin City has a population of 3,000 and is ethnically diverse with large East Asian, Mexican, and Black populations. Many families live in public housing apartment buildings. The population in the Canal area is largely Hispanic, with many households residing in apartment units. San Rafael has asserted to the Federal Government that this population is significantly undercounted by the U.S. Census due to the high percentage of illegal immigrants. They assert that the 6.6% of the county-wide population listed as below the poverty line is both under-reported and heavily concentrated in the Canal area.